Idiopathic dilated cardiomyopathy is a disease of the heart muscle resulting in enlargement and weakening of the heart, congestive heart failure, rhythm disturbances, and death, usually within 5 years. Several lines of evidence suggest that this disease may be immune-mediated, and that it might potentially be remediable with anti-inflammatory therapy. Between 1982 and 1986, approximately 200 patients were screened for this study, and approximately 150 were admitted to the Clinical Center for comprehensive inpatient evaluation. Of these, 102 were randomized to either a prednisone treatment group or a control group, and all were brought back for periodic inpatient reevaluations, including serial transvenous endomyocardial biopsies. The final inpatient reevaluations were completed in late 1988. Analysis of results demonstrates that prednisone does improve left ventricular function in a subset of idiopathic dilated cardiomyopathy patients, specifically, those with fibroblastic infiltration of the myocardium. The study continues to provide valuable longitudinal natural history data concerning this disease, and has permitted new estimations of the mortality of the disease, the prognostic determinants, the indications for cardiac transplantation, and the clinical value of endomyocardial biopsy, cardiac gallium screening, and other techniques. The study additionally has provided new insights into the pathophysiology of the disease, the role of inflammation, and the role of neurohumoral factors.